Heteroporphyrin nanotubes and composites

ABSTRACT

Heteroporphyrin nanotubes, metal nanostructures, and metal/porphyrin-nanotube composite nanostructures formed using the nanotubes as photocatalysts and structural templates, and the methods for forming the nanotubes and composites.

This application is a divisional application of the prior-filedcopending U.S. nonprovisional patent application Ser. No. 11/001,468,filed on Dec. 1, 2004, and claims priority benefit therefrom. Thisprior-filed copending application is hereby incorporated by reference.

The United States Government has rights in this invention pursuant toDepartment of Energy Contract No. DE-AC04-94AL85000 with SandiaCorporation.

BACKGROUND OF THE INVENTION

This invention relates to heteroporphyrin nanostructures and compositesthereof with incorporated metals. It is known that certain porphyrins inaqueous solution can form J-aggregates (off-set stacked molecules withaligned transition dipoles). The coherent coupling of the transitiondipoles of porphyrin monomers gives rise to aggregate absorption bandsthat are significantly red-shifted relative to the monomer bands.Additional optical properties of the aggregates include giant resonancelight scattering, which imparts intense color to a colloidal solution ofsuch materials when viewed at an angle to the direction of propagation,and possible nonlinear optical properties.

These aggregates are typically in the form of fractal objects ornanoscale flecks of the aggregated porphyrin. An example of such fractalstructures is reported in N. Micali et al., “Fractal Structures in Homo-and Heteroaggregated Water Soluble Porphyrins,” J. Phys. Chem. B 104,9416-9420 (2000).

The lack of linearity in the structure of these aggregates is adisadvantage for many possible applications of such materials where awell-defined morphology, such as nanotubes or nanorods, is desirable.This invention comprises a method for making nanotubular heteroporphyrinJ-aggregates possessing a novel linear configuration and the nanotubesmade thereby. The nanotubes comprise at least two types of porphyrinmoieties with at least one positively charged porphyrin moiety and atleast one negatively charged porphyrin moiety. It further comprises thenanostructures resulting from the metallization of these porphyrinnanotubes and the method of making them.

The formation of nanorods, fibers, tubules, helical ribbons, and sheetsusing a single porphyrin type as the molecular subunit has beenpreviously reported, for example, by Furhop et al., “Micellar Rods andVesicular Tubules Made of 14′″, 16′″-Diaminoporphyrins,” J. Am. Chem.Soc. 115 (1993) p. 11036-11037, Siggel et al., “Photophysical andPhotochemical Properties of Porphyrin Aggregates,” Ber. Bunsenges. Phys.Chem. 100 (1996) p. 2070-2075; Schwab et al., “Porphyrin Nanorods,” J.Phys. Chem B 107 (2003) p. 11339-11345; and Rotomskis et al.,“Hierarchical Structure of TPPS₄ J-Aggregates on Substrate Revealed byAtomic Force Microscopy,” J. Phys. Chem. B 108 (2004) p. 2833-2383.

SUMMARY OF THE INVENTION

This invention comprises heteroporphyrin nanotubes and metalnanostructures and the method for making heteroporphyrin nanotubes andusing the nanotubes for making the metal nanostructures andmetal/porphyrin-nanotube composites.

Additional objects, advantages, and novel features of the invention willbecome apparent to those skilled in the art upon examination of thefollowing description or may be learned by practice of the invention.The objects and advantages of the invention may be realized and attainedas particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate an embodiment of the present inventionand, together with the description, serve to explain the principles ofthe invention.

FIG. 1 illustrates the structure of some of the porphyrins and porphyrinprecursors described herein: (a)meso-tetrakis(4-sulfonatophenyl)porphyrin, (b)Sn(IV)meso-tetrakis(4-pyridyl)porphyrin, (c) VO or TiOmeso-tetrakis(4-pyridyl)porphyrin, (d) Fe(III) orCo(III)meso-tetrakis(4-pyridyl)porphyrin, and (e)Sn(IV)meso-tetrakis(3-pyridyl)porphyrin. In (b), (d), and (e), X and Clrepresent a bound ligand such as, for example, halide, hydroxide, andwater.

FIG. 2 consists of two transmission electron micrographs (TEMs) showing(a) a side view of a porphryin nanotube and (b) an end-on view of aporphyrin nanotube showing the hollow interior.

FIG. 3 is a TEM showing the uniformity of size and shape of theporphyrin nanotubes.

FIG. 4 consists of three TEMs showing (a) a porphyrin nanotube, (b) agold metal nanostructure consisting of a nanorod and a continuouslyattached nanoball that has been grown using the nanotube as a structuraltemplate, and (c) a gold nanorod with a continuously attached nanoball(a ball-and-rod nanostructure) from which the porphyrin template hasbeen removed by treatment with NaOH at pH 10.

FIG. 5 consists of TEMs showing metallic platinum nanostructures wherethe platinum has been deposited (a) on the surface of a porphyrinnanotube, (b) within the hollow interior of the porphyrin nanotube, and(c) both within the hollow interior of the porphyrin nanotube and on theouter surface of the porphyrin nanotube.

FIG. 6 consists of TEMs showing metallic nanostructures where metaldeposited within the hollow interior of the porphyrin nanotube differsfrom the metal deposited on the surface of the nanotube. (a) Pt within aporphyrin nanotube and Au on the surface of the porphyrin nanotube. (a)Au within a porphyrin nanotube and Pt on the surface of the porphyrinnanotube.

DETAILED DESCRIPTION OF THE INVENTION

This invention comprises heteroporphyrin nanotubes, metalnanostructures, and metal/porphyrin-nanotube composite nanostructuresformed using the nanotubes as photocatalysts and structural templates,and the methods for forming these structures.

Functional self-assembled materials with well-defined shapes anddimensions are of great current interest, especially for applications inelectronics, photonics, light-energy conversion, and catalysis. Inbiological systems, tetrapyrroles such as porphyrins and chlorophyllsare often organized into nanoscale biological superstructures thatperform light-harvesting and energy- and electron-transfer functions.One example is the light-harvesting rods of the chlorosomes ofgreen-sulfur bacteria, which are composed entirely of aggregatedbacteriochlorophyll. Because of their desirable functional properties,porphyrins and other tetrapyrroles are attractive building blocks forfunctional nanostructures.

This invention comprises porphyrin nanotubes that can be prepared byionic self-assembly of two oppositely charged porphyrins in aqueoussolution. The charged porphyrins assemble by rapid ionic self-assemblyto form multimolecular subunits. The multimolecular subunits thenaggregate more slowly to form the nanotubes. The nanotubes of thisinvention represent a new class of porphyrin nanostructures whosestructural and functional characteristics can be varied by properselection of the molecular building blocks (tectons) used to form thenanostructure.

The porphyrin nanotubes are formed by ionic self-assembly of oppositelycharged porphyrin molecules. Some examples of suitable porphyrins areshown in FIG. 1. The electrostatic forces between these porphyrinconstituents promote the structural stability of these nanostructures.The multimolecular subunits formed from the combination of bothpositively and negatively charged porphyrins, includingmetalloporphyrins, provide the basic building blocks for the formationof the nanotube. Additional intermolecular interactions that cancontribute to the structural stability include van der Waals forces,hydrogen bonding, axial coordination, and other weak intermolecularinteractions. Molecular recognition of the complementary arrangements ofthe charges and H-bond donor/acceptors of the porphyrin guide theformation of the multimolecular subunits. The pH of the solution isimportant for providing the appropriate level of protonation ofsubstituents of the porphyrins and of ligands bound to the metal ion inmetalloporphyrins, such as, for example, hydroxyl groups, pyridylgroups, and bound water molecules. Formation of multimolecular subunitsand their subsequent self-assembly into the nanotubes can be controlledby proper selection of the specific porphyrin constituents, the pH, andthe solvent. Some examples of suitable metalloporphyrins includeCo(III)meso-tetrakis(4-pyridyl)porphyrin,Fe(III)meso-tetrakis(4-pyridyl)porphyrin,Sb(IV)meso-tetrakis(4-pyridyl)porphyrin, Sn(IV)meso-tetrakismeso-tetrakis(4-pyridyl)porphyrin, TiOmeso-tetrakis(4-pyridyl)porphyrin, VO meso-tetrakis(4-pyridyl)porphyrin,Co(III)meso-tetrakis(3-pyridyl)porphyrin,Fe(III)meso-tetrakis(3-pyridyl)porphyrin,Sb(IV)meso-tetrakis(3-pyridyl)porphyrin andSn(IV)meso-tetrakis(3-pyridyl)porphyrin, TiOmeso-tetrakis(3-pyridyl)porphyrin, and VOmeso-tetrakis(3-pyridyl)porphyrin.

The porphyrin nanotubes of this invention are hollow structurespossessing uniform size and shape for a given pair of cationic andanionic porphyrins, as shown in transmission electron micrographs (TEMs)in FIGS. 2 and 3. FIG. 2 shows the uniformity of wall thickness and thehollow interior of a single tube. FIG. 3 shows the high degree ofuniformity of size for a large number of nanotubes generated in a singlesynthetic run. The nanotubes are photocatalytic, mechanically responsiveand adaptive to light, and they have other interesting electronic andoptical properties, some of which mimic properties of the chlorosomalnanorods.

In a typical embodiment, porphyrin nanotubes are formed by mixingaqueous solutions of the two or more porphyrin species with someporphyrin species being positively charged and others being negativelycharged. The solvent of the aqueous solution can be water or its mixturewith some other polar organic solvents, such as methanol, ethanol,acetonitrile, tetrahydrofuran, dimethyl formamide, and dimethylsulfoxide.

In one embodiment, 9 mL of freshly acidified H₄TPPS₄ ²⁻ solution (10.5μM H₄TPPS₄ ²⁻, 0.02 M HCl) was mixed with 9 mL ofSn(IV)tetrakis(4-pyridyl)porphyrin (SnTPyP²⁺) dichloride in water (3.5μM SnTPyP²⁺). The mixture, which had a pH of 2, was left in the dark atroom temperature for 72 h. While porphyrins of a single charge typeexhibit negligible aggregation under these conditions (pH 2) within 5hours, the mixture of both positively and negatively charged porphyrinsimmediately forms colloidal aggregates, which over a period of timeself-organize into nanotubes with a high yield (approximately 90%). Thenanotubes resulting from this synthetic process are illustrated in FIG.2.

Transmission electron microscope (TEM) images of the porphyrin nanotubes(FIGS. 2 and 3) reveal that they are micrometers in length and havediameters in the range of 50-70 nm with approximately 20-nm thick walls.Images of the nanotubes caught in vertical orientations (FIG. 2( b))confirm a hollow tubular structure with open ends. Fringes with1.7-1.8-nm spacing are seen both in end-on views and at the edges of thenanotubes in TEM images and probably originate from the heavy tin andsulfur atoms in the porphyrin stacks. The fringes seen in the TEMimages, combined with the optical spectral results discussed below, areconsistent with a structure composed of stacks of offset J-aggregatedporphyrins (individual porphyrins are approximately 2×2×0.5 nm) in theform of cylindrical lamellar sheets. The lamellar structure would besimilar to an architecture proposed for the stacking ofbacteriochlorophyll molecules in the chlorosomal rods. X-ray diffractionstudies exhibit peaks in the low- and high-angle regions with peakwidths suggesting moderate crystallinity.

The composition of the nanotubes was determined by UV-visible absorptionspectroscopy and energy dispersive X-ray (EDX) spectroscopy. Thefiltered nanotubes were dissolved at pH 12 and the ratio of theporphyrins was determined by spectral simulation using extinctioncoefficients for H₂TPPS₄ ⁴⁻ (ε₅₅₂=5500 M⁻¹-cm⁻¹) and Sn(OH)₂TPyP(ε₅₅₁=20200 M⁻¹-cm⁻¹), giving an approximate molar ratio of 2.4 H₂TPPS₄⁴⁻ per Sn(OH)₂TPyP. EDX measurements of the S:Sn atomic ratio of theporphyrin tubes on the TEM grids also indicate a molar ratio of between2.0 and 2.5. The observed ratio of the two porphyrins in the tubes(2.0-2.5) can be related to the charges of the porphyrin species presentat pH 2. As shown by titrations monitored by UV-visible spectroscopy,the porphyrin species present at pH 2 are H₄TPPS₄ ²⁻ and a mixture ofSn(OH)₂TPyP⁴⁺ and Sn(OH)(H₂O)TPyP⁵⁺ (formed by protonation of thepyridine substituents of Sn(OH)₂TPyP and, in the latter species, by theadditional replacement of one OH⁻ axial ligand with H₂O at low pH). Theformation of the nanotubes critically depends on the pH (e.g., notformed at pH 1 or 3), as expected because the charge balance of theionic porphyrins depends on their protonation state. EDX spectra of thenanotubes also showed no evidence of significant amounts of Cl (or Iwhen HI was used in place of HCl), precluding the presence of chlorideas counter ions, axial ligands, or salt bridges. As expected for ananostructure formed by ionic self-assembly, the same ratio ofporphyrins (2.0-2.5) is observed in the nanotubes regardless of theinitial ratio of the two porphyrins in solution.

By altering the molecular structure of the porphyrins, the dimensions ofthe nanotubes can be controlled. For example, by using Sntetra(3-pyridyl)porphyrin instead of Sn tetra(4-pyridyl)porphyrin,nanotubes with significantly smaller average diameters were obtained (35nm instead of 60 nm). Changing the tin porphyrins subtly repositions thecharge centers and the associated H-bond donor atoms on the pyridiniumrings, changing the inter-porphyrin interactions sufficiently to alterthe diameter while still allowing the tubes to form. Nanotubes are notproduced when the 2-pyridyl porphyrin is used, presumably because thelocation of the functional nitrogen atom is changed too drastically.Axial ligation of a porphyrin is also important as tubes are obtainedwhen the Sn(IV) complex is replaced with other potentiallysix-coordinate metal ions, for example, Fe³⁺, Co³⁺, TiO²⁺, VO²⁺, but notwhen a metal that does not add axial ligands, for example, Cu²⁺, or themetal-free porphyrin is used. These results illustrate the possibilityof achieving a control over the structure of the nanotubes by suitablestructural modifications of the porphyrins, including variation of theperipheral substituents of the porphyrin, the metal contained in theporphyrin core, and the nature of the axial ligands.

The nanotubes exhibit interesting and potentially useful properties. Forexample, the porphyrins in the nanotubes are stacked in a manner thatgives UV-visible absorption bands at 496 and 714 nm that are red-shiftedfrom the corresponding bands of the monomeric porphyrins. These bandsindicate formation of J-aggregates similar to those of H₄TPPS₄ ²⁻ andother porphyrins, but the bands of the nanotubes are broader. As aresult, the nanotubes exhibit intense resonant light scattering fromthese J-aggregate bands, making the nanotube suspension appear brightgreen under intense white light illumination, but light greenish yellowin weak transmitted light. In addition, the strong fluorescence of theporphyrin monomers is almost entirely quenched in the nanotubes.

A potentially useful property of the nanotubes is their ability torespond mechanically to light illumination. Even though they are stablefor months when stored in the dark, irradiation of a suspension of thetubes for just five minutes using incandescent light from a projectorlamp (800 nmol-cm⁻²-s⁻¹) results in TEM images showing rod-likestructures instead of tubes. This response to light is reversible, asthe tubes reform (self-heal) when left in the dark. The switch fromtubular to rod-like structures in the TEM images suggests a softening ofthe tube walls and a collapse of the tube structure, perhaps as a resultof photoinitiated intermolecular electron transfer that disrupts thecharge balance and hence the rigidity of the structure of the ionicsolid.

Some metalloporphyrins, for example, the Sn(IV) porphyrins, are known tobe good photocatalysts in homogeneous solutions. Porphyrin nanotubeswith constituent tin porphyrins exhibit useful photocatalytic activity.This has been demonstrated in two embodiments of this invention wherethe photoreduction of aqueous metal complexes has been used to formmetal nanostructures using the porphyrin nanotubes as structuraltemplates. The two types of metal salts employed to demonstrate thisinvention in these two embodiments are Au(I) complexes and Pt(II)complexes. The photocatalytic reduction reaction mediated by Sn(IV)porphyrins in homogeneous solutions is described for reduction of aAu(I) complex by the following simplified cyclic reactions:(Sn porphyrin)+light

(Sn porphyrin)*(Sn porphyrin)*+ED

(Sn porphyrin)•+ED_(ox)(Sn porphyrin)•+M⁺

(Sn porphyrin)+M⁰where ED=electron donor, * indicates a photoexcited state of theporphyrin, • indicates the reduced porphyrin or porphyrin radical ion,ED_(ox) is the oxidized form of the electron donor, and M⁺ and M⁰indicate the metal ion and reduced metal, respectively. Typical examplesof electron donors include but are not restricted to ascorbic acid,ethylene diamine tetra-acetic acid (EDTA) in varying degrees ofprotonation, triethylamine, triethanolamine, benzenethiol,2-mercaptoethanol, and nitrite ions.

In one embodiment, Au(I) thiourea or Au(I) thiosulfate complexes wereused with ascorbic acid as the electron donor (ED). These reductions arepredominately photocatalytic, unlike those for many other gold complexesthat may also involve chemical and photochemical reduction. Thephotochemical reduction is prevented by the higher stability of theAu(I) complexes relative to Au(III) complexes and their transparency tovisible light. Autocatalytic growth of gold is negligible.

The Sn-porphyrin-containing nanotubes used in the metallizationreactions were prepared as described above. A transmission electronmicroscope (TEM) image of the porphyrin nanotubes before metaldeposition is shown in FIG. 4( a). The tubes can be several micrometersin length and have diameters typically in the range of 50-70 nm withapproximately 20-nm thick walls. When the nanotubes are used tophotoreduce the positively charged Au(I)-thiourea complex, the metal isdeposited exclusively within the hollow interior of the nanotubes,forming a continuous polycrystalline gold nanowire that is of the samediameter as the tube core, as shown in FIG. 4( b). The nanowires can beterminated at one end of the nanotube with a gold ball that is generallyof larger diameter than the tube. When the porphyrin nanotubes aredissolved by raising the pH, the gold wire and ball remain intact asshown in FIG. 4( c). The diameter of the nanorod is determined by theinner diameter of the hollow porphyrin nanotube. The diameter of theball can vary from about the same diameter as the nanotube to muchlarger. Nanoball diameters up to 800 nm have been demonstrated andlarger-diameter nanoballs are possible, depending on the reactionconditions and growth time after the nanogrowth emerges from theinterior of the nanotube.

Selection of a negatively charged gold complex can produce a differentnanostructure. In one embodiment, the reduction of the negativelycharged Au(I) thiosulfate complex produces gold particles primarily onthe outer surfaces of the tubes. These results show that theelectrostatic and other interactions between the complex and the tubesurfaces can control where the metal is deposited and the location ofthe metal deposition is different for the two oppositely chargedcomplexes. Directional electron/energy transport within the tube wallscan also play a role in determining where the metal is deposited.

In one embodiment, gold solutions for the photocatalytic deposition ofgold were freshly prepared by reducing Au(III) to Au(I) with thiourea(V. Gaspar, A. S. Mejerovich, M. A. Meretukov, and J. Schmiedl,Hydrometallurgy, 1994, 34, 369). Typically, 17 mg of thiourea powder wasslowly added to 1 ml of HAuCl₄ solution (20 mM) and the mixture agitatedto dissolve the thiourea. After a few minutes, the yellow Au(III)solution turned into a transparent and colorless solution ofAu(I)(thiourea)_(x) complex. To grow the Au nanostructure, 50 μl ofAu(I)(thiourea)_(x) solution (20 mM Au) and 50 μl of ascorbic acidsolution (0.2 M) were added to a 2-ml glass vial containing 1 ml of thenanotube colloidal suspension (SnTPyP²⁺ concentration 1.75 μM). Thereaction mixture was swirled to homogenize the solution, placed in aglass water bath to control the temperature, and then irradiated withincandescent light (800 nmol cm⁻² s⁻¹) for 8 minutes. No gold depositionwas observed in control experiments without light exposure or in theabsence of the porphyrin nanotubes.

The continuous nature of the nanowire and the formation of the ball atthe end of the tube suggest a novel mechanism for its formation by thenanotube. The continuity of the wire implies a single nucleation site onthe inner surface with mobile electrons generated in the tube flowinginto the wire at that site. Thus, under illumination, the tube acts as aphotoelectrochemical cell, charging up the growing nanowire at apotential negative enough to reduce the gold complex at gold surfacesaccessible to the complex. When the wire reaches the end of the tube itexpands into the ball where most of the electrons will collect. TheJ-aggregate composition of the tube walls indicates strong electroniccoupling of multiple porphyrin subunits, which might be expected tofacilitate electron transport necessary to grow the nanowire.

In one embodiment of platinum nanostructures formed in this invention,platinum metal can be grown onto the nanotube surfaces as illustrated inFIG. 5( a). Photocatalytic initiation of growth by photoreduction of theplatinum complex occurs with the nanotubes to produce small Pt seednanoparticles. These seed particles decorate mainly the outer surfacesof the porphyrin nanotubes, as shown in FIG. 5( a). Normally, fastautocatalytic reduction of Pt occurs after the seed particle reaches acertain size, producing a Pt dendrite, and some nascent Pt dendrites arevisible in high magnification TEM images. At higher Pt concentrations,it is possible to grow one or more Pt dendrites or columns of Ptnanoparticles within the hollow interior of the nanotube, as shown inFIG. 5( b). FIG. 5( c) shows Pt nanostructures both within the hollowinterior and on the outer surface of the nanotube. The concentrations ofthe Pt complex and ascorbic acid were 0.1 and 1 mM, respectively, for(a) and 1 and 10 mM, respectively, for (b) and (c). Exposure times were15 minutes for (a) and (b) and 35 minutes for (c). The concentrations ofthe nanotubes were the same for both reactions. Composites with bothfilled cores and large platinum dendrites on the outer surface can alsobe produced at high Pt ion concentration and longer light exposuretimes.

In one embodiment for the deposition of platinum, K₂PtCl₄ stock solution(20 mM Pt) and ascorbic acid stock solution (0.2 M) were added to a 2-mlglass vial containing 1 ml of the nanotube colloidal suspension(SnTPyP²⁺ concentration 1.75 μM). The reaction mixture was swirled tohomogenize the solution, placed in a glass water bath to control thetemperature, and then irradiated with incandescent light (800 nmol cm⁻²s⁻¹). When a low concentration of platinum salt (0.1 mM) was used in thereaction, 5 μl of K₂PtCl₄ stock solution and 5 μl of ascorbic acid stocksolution were added and the light exposure time was 15 min. When a highconcentration of platinum salt (1 mM) was employed in the reaction, 50μl of K₂PtCl₄ stock solution and 50 μl of ascorbic acid stock solutionwere added and the light exposure time was 15 or 35 min. The porphyrinconcentration was essentially the same for all the reactions (1.75 μMSnTPyP²⁺).

It is possible to construct metallic nanostructures that comprise onemetal within the nanotube and different metal on the outer surface ofthe nanotube. In one embodiment, a Pt core is formed within the nanotubeand dendritic Au is grown on the surface of the nanotube. This wasaccomplished by growing the Pt as follows: 100 μl K₂PTCl₄ solution (20mM inPt) and 100 μl of 0.2 M ascorbic acid solution were added to a 2-mlglass vial containing 1 ml of the nanotube colloidal suspension(SnTPyP²⁺ concentration 1.75 μM). The reaction mixture was mixed tohomogenize the solution, placed in a water bath to control thetemperature, and irradiated with incandescent light (800 nmol cm⁻² s⁻¹)for 11 min. The suspension was centrifuged at 2000 rpm for 2 minutes.The supernatant liquid was removed and 1 ml of 0.01 M HCl was added.Following mixing to obtain a relatively homogeneous suspension, 100 μlof 0.2M ascorbic acid solution and 100 μl of 20 mM Au(I) thiourea_(x)complex were added. The reaction mixture was mixed to homogenize thesolution, placed in a water bath to control the temperature, andirradiated with incandescent light (800 nm cm⁻² s⁻¹) for 9 minutes. Thestructure resulting from such growth procedures is shown in FIG. 6( a).

Composite structures with Au interior wires and Pt nanoparticles on theouter surface of the nanotube can be made, as shown in FIG. 6( b). Inone such embodiment, 50 μl of 20 mM Au(I) thiourea_(x) complex and 100μl of 0.2 M ascorbic acid were added to a 2-ml glass vial containing 1ml of the nanotube colloidal suspension (SnTPyP²⁺ concentration 1.75μM). The reaction mixture was mixed to homogenize the solution, placedin a water bath to control the temperature, and irradiated withincandescent light (800 nmol cm⁻² s⁻¹) for 6 min. The suspension wascentrifuged at 1500 rpm for 1 min. The supernatant liquid was removed.One milliliter of Pt nanoparticle colloidal suspension preparedaccording to the literature (Brugger, P. A.; Cuendet, P.; Gratzel, M. J.Am. Chem. Soc. 1981, 103, 2923-2927) and 10 μl of 1 N HCl were added;the resulting suspension was mixed to achieve relative homogeneity Themixed suspension was kept in the dark overnight while the Ptnanoparticles reacted and adsorbed onto or bound to the porphyrinnanotube outer surface.

The preceding embodiments illustrate this invention using Au(I) andPt(II) ions. This invention is applicable to a range of other metal thathave reduction potentials compatible with the potential provided by thephotoreduced metalloporphyrin incorporated within the nanotube. Examplesinclude but are not restricted to Ag(I) ions, Pd ions, Ni ions, Cu ions,Co ions, Fe ions, Rh ions, Pb ions, oxides of Ru, Cr, and U, and Se(IV),for example, as in SeO₃ ²⁻.

It should be apparent that there are many modifications possible withthis invention. It is intended that the scope of the invention bedefined by the appended claims.

1. A metal nanostructure, comprising: a porphyrin nanotube with a hollowinterior, the porphyrin nanotube having an outer surface; and a metalnanorod within the hollow interior, the metal nanorod having a first endand a second end.
 2. The metal nanostructure of claim 1, furthercomprising a metal ball attached to the first end of the metal nanorod.3. The metal nanostructure of claim 1, further comprising a metal filmon the outer surface of the porphyrin nanotube.
 4. The metalnanostructure of claim 1, further comprising a metal particle on theouter surface of the porphyrin nanotube.
 5. The metal nanostructure ofclaim 1, wherein the porphyrin nanotube comprises a metalloporphyrinwith a coordinated metal ion.
 6. The metal nanostructure of claim 5,wherein the coordinated metal ion is selected from the group consistingof antimony, cobalt, iron, tin, titanium, and vanadium.
 7. The metalnanostructure of claim 5, wherein the metalloporphyrin is selected fromthe group consisting of Co(III)meso-tetrakis(4-pyridyl)porphyrin,Fe(III)meso-tetrakis(4-pyridyl)porphyrin,Sb(IV)meso-tetrakis(4-pyridyl)porphyrin, Sn(IV)meso-tetrakismeso-tetrakis(4-pyridyl)porphyrin, TiOmeso-tetrakis(4-pyridyl)porphyrin, VO meso-tetrakis(4-pyridyl)porphyrin,Co(III)meso-tetrakis(3-pyridyl)porphyrin,Fe(III)meso-tetrakis(3-pyridyl)porphyrin,Sb(IV)meso-tetrakis(3-pyridyl)porphyrin andSn(IV)meso-tetrakis(3-pyridyl)porphyrin, TiOmeso-tetrakis(3-pyridyl)porphyrin, and VOmeso-tetrakis(3-pyridyl)porphyrin.
 8. The metal nanostructure of claim1, wherein the porphyrin nanotube comprises J-aggregated porphyrins. 9.The metal nanostructure of claim 1, wherein the metal nanostructurecomprises at least one material selected from the group consisting ofAu, Pt, Pd, Ag, Fe, Co, Ni, Cu, Rh, Pb, Se, CrO₂, RuO₂, and UO₂.
 10. Ametal nanostructure, comprising: a porphyrin nanotube with a hollowinterior, the porphyrin nanotube having an outer surface; a first metalnanorod within the hollow interior, the first metal nanorod having afirst end and a second end; and a second metal deposit on the outersurface.
 11. The metal nanostructure of claim 10, further comprising ametal ball attached to the first end of the first metal nanorod.
 12. Themetal nanostructure of claim 10, wherein the first metal nanorod and thesecond metal deposit comprise at least one material selected from thegroup consisting of Au, Pt, Pd, Ag, Fe, Co, Ni, Cu, Rh, Pb, Se, CrO₂,CrO₃, RuO₂, and UO₂.
 13. The metal nanostructure of claim 10, whereinthe first metal nanorod and the second metal deposit contain at leastone different metallic element.
 14. The metal nanostructure of claim 10,wherein the porphyrin nanotube comprises a metalloporphyrin with acoordinated metal ion.
 15. The metal nanostructure of claim 14, whereinthe coordinated metal ion is selected from the group consisting ofantimony, cobalt, iron, tin, titanium, and vanadium.
 16. A metalnanostructure comprising a ball-and-rod nanostructure, made by a methodcomprising: preparing a solution of a porphyrin nanotube, a metalprecursor, and an electron donor, the porphyrin nanotube having a hollowinterior, a first end, and a second end; illuminating the solution witha light beam at a wavelength that produces a photoexcitation of theporphyrin nanotube, thereby making a photoexcited porphyrin-nanotubetemplate; photoreducing the metal precursor by means of the photoexcitedporphyrin-nanotube template to form a metal nanorod inside the hollowinterior of the porphyrin nanotube, photoreducing the metal precursor bymeans of the photoexcited porphyrin-nanotube template to form a metalball continuously attached to the metal nanorod at the first end of theporphyrin nanotube to form the ball-and-rod nanostructure; and removingthe porphyrin nanotube from the ball-and-rod nanostructure using a basicsolution.
 17. The metal nanostructure of claim 16, wherein a diameter ofthe metal nanorod is between about 5 nm and about 200 nm and a diameterof the metal ball is between the diameter of the metal nanorod and 1micrometer.